Traditional metal containers, such as food and beverage cans, are known as three piece cans. These cans were formed of three separate components; a sidewall which is formed by bending a flat sheet of metal into a cylindrical shape and applying a weld along the seam to retain that shape; a top end and a bottom end. While successful, these three piece cans were slow to manufacture and contained several potential sources of leakage (side seam and top and bottom ends).
In the past decade, a new container design has been replacing the traditional three piece can. This new design is known as the "two piece can". A two piece can is formed from a first piece which is worked so as to form a unitary sidewall and bottom and a second piece which forms the top end of the container. This design eliminates several potential sources of leakage, allows for the entire surface to be printed and reduces the amount of metal used in the formation of the container.
Two piece cans are generally formed of either tin plated steel or more preferably aluminum.
The process for forming the two piece containers is known as the "drawing and ironing" process. In the drawing step, a flat sheet of metal is formed into a shallow cuplike piece similar in appearance to a petri dish. This cup is then placed into an ironer which has an outer die shaped to reflect the desired outer dimensions of the container and an inner, moveable punch configured to match the desired inner dimensions of the container. The punch is moved against the bottom of the cup one or more times to lengthen the cup, reduce its wall thickness and form it into the desired shape.
During the drawing and ironing steps, a metalworking fluid is required to prevent rupturing of the metal and to prevent the metal from sticking to the tooling. More particularly, a metalworking fluid primarily having lubricating properties is required during the drawing process and a metalworking fluid having primarily cooling properties is required for the ironing process. Various metalworking fluids have been used in these processes. However, most of these fluids are useful on one of the two process steps but not both, thereby requiring the use of two different fluids which require the use of separate sump collectors and waste treatment systems.
Additionally and perhaps more importantly, the current metalworking fluids tend to leave a residue on the tooling and container surface. The buildup of the residue on the tooling reduces the life of the tooling, causes streaking of the metal surface and increases downtime of the machines for cleaning. The residue left on the container surface cannot be completely removed by a post formation washing process. The residue that remains is thought to bond with the metal surface. It is believed that this remaining residue interacts with the contents of the container to create an off taste in the product. This problem is particularly serious in beer where the residue gives the beer an old or stale taste.
The present invention overcomes the shortcomings of the present metalworking fluids by providing a metalworking fluid that is useful in both the drawing and ironing processes and creates little, if any, formation of residue upon the tooling and container.